In recent years, a biological signal processor that processes a signal based on subject's biological displacement (such as a respiration, a heartbeat, a pulse, and a body motion) has been proposed. The biological signal processor is, for example, applied to a sleep state measuring apparatus that measures a subject's sleep state. In the sleep state measuring apparatus, although starting the measurement of the sleep state requires a start command to the apparatus, a problem arises in that since sleeping is everyday behavior, the subject tends to forget the command operation (such as switch operation). When the biological signal processor is used, without any special operation on the sleep state measuring apparatus by the subject, it is possible to determine that the subject is in bed thereby performing a sleep state measurement. For example, Japanese Patent Application Publication No. 2008-259745 discloses a technique in which a mat-type sensor installed under bedding is used so as to acquire a signal based on biological displacement such as a body motion, a respiration, and a heartbeat from an output of the sensor. The mat-type sensor is a sensor that uses air or water for detecting internal pressure variation of the mattress.